1. Field of the Invention
The present disclosure relates to a developing device including an developer bearing member configured to bear a developer and a supply roller configured to supply the developer to the developer bearing member. This developing device can be used for an electrophotographic apparatus such as a printer and a copying machine.
2. Description of the Related Art
In image forming apparatuses such as printers using the electrophotographic image forming method (the electrophotographic process), an electrophotographic photosensitive member (hereinafter referred to as a “photosensitive member”) as an image bearing member is evenly charged, and the charged photosensitive member is selectively exposed, whereby an electrostatic latent image is formed on the photosensitive member. The electrostatic latent image formed on the photosensitive member is visualized as a toner image by toner as a developer. Then, the toner image formed on the photosensitive member is transferred to a recording material such as recording paper and a plastic sheet, and further, heat and a pressure are applied to the toner image transferred onto the recording material to fix the toner image to the recording material, thereby performing image recording.
Generally, such image forming apparatuses require a replenish of the developer and maintenance of various kinds of process units. Assembling, for example, the photosensitive member, a charging unit, a developing unit, and a cleaning unit in a frame member to form a cartridge, and handling them as a process cartridge detachably attachable to a main body of the image forming apparatus are put into practical use to facilitate the developer replenish work and the maintenance of the various kinds of process units. According to the process cartridge method, it is possible to provide image forming apparatuses having excellent usability.
Further, in recent years, color image forming apparatuses, which form a color image using developers of a plurality of colors, have been widely used. As one type of color image forming apparatus, an in-line type image forming apparatus has been known. The in-line type image forming apparatus is configured in such a manner that photosensitive members corresponding to the respective image forming operations using developers of a plurality of colors are arranged in line along a surface movement direction of a member to be transferred to which a toner image is transferred. As the in-line type color image forming apparatus, there is an image forming apparatus including the plurality of photosensitive members arranged in line in a direction (for example, the horizontal direction) intersecting with the vertical direction (the direction of gravity). The in-line method is a desirable image forming method because, for example, it can easily satisfy demands such as speed-up of the image forming speed and development into a multifunction printer.
Further, as the in-line type image forming apparatus including the plurality of photosensitive members arranged in line in the direction intersecting with the vertical direction, Japanese Patent Application Laid-Open No. 2003-173083 discusses an image forming apparatus including the plurality of photosensitive members arranged below an intermediate transfer member as a member to be transferred or a recording material bearing member that conveys a recording material as a member to be transferred.
In a case where the photosensitive members are arranged below the intermediate transfer member or the recording material bearing member, for example, the fixing device and the developing device (or the exposure device) can be arranged away from each other with the intermediate transfer member or the recording material bearing member sandwiched therebetween within the main body of the image forming apparatus. This can provide such a benefit that the developing device (or the exposure device) becomes less affected by the heat of the fixing device.
On the other hand, in a case where the photosensitive members are arranged below the intermediate transfer member or the recording material bearing member as described above, the developing device may have to supply the developer to the developer bearing member and a supply roller against the gravity.
In this supply of the developer to the supply roller, it is most efficient to convey toner to a position immediately after a contact region (above the nip portion) between the developer bearing member and the supply roller in the rotational direction of the supply roller. This is because the supply roller includes an elastic layer having a plurality of cells on the outer circumference thereof, and the position immediately after the nip portion has such an effect that the cells are released from a pressure of the nip portion and are opened according thereto. The cells suck air at this time, thereby sucking the developer. Therefore, it is desirable to directly transmit the toner to this suction portion to make the supply roller hold the developer efficiently.
However, in the developing device that supplies the developer against the gravity, it is difficult to transmit a large amount of toner to the suction portion with a simple structure. Further, even if the toner can be transmitted to the vicinity of the suction portion, apart thereof can be sucked, but most of the toner is returned to, for example, a toner storage unit according to a rotation of the supply roller.
Therefore, Japanese Patent Application Laid-Open No. 2003-173083 discusses the technique for supplying the developer at the lower portion of the supply roller. In the technique discussed in Japanese Patent Application Laid-Open No. 2003-173083, the supply roller rotates upwardly at an abutment portion between the developing roller (the developer bearing member) and the supply roller (the developer supply member). Then, a toner receiving member is disposed below the supply roller. One end of a receiving sheet is attached to the toner receiving member, and this receiving sheet is brought into contact with the lower portion of the supply lower at an appropriate linear pressure.
In this way, Japanese Patent Application Laid-Open No. 2003-173083 discusses the method for bringing the receiving sheet into contact with the lower portion of the supply roller. According to this method, this receiving sheet prevents the developer attached to the supply roller from falling due to the gravity, prevents a reduction in the developer that can be supplied to the developing roller, and thereby prevents a reduction in the density of a solid image.
However, according to the developer supply method discussed in Japanese Patent Application Laid-Open No. 2003-173083, when images are formed at a low printing ratio in a row, the developer between the supply roller and the receiving sheet may stay for a long time without being consumed. In this case, the developer continues being supplied to between the supply roller and the receiving sheet according to a rotation of the supply roller, and the developer is coagulated there. As a result, the toner supply amount from the supply roller to the developing roller may become uneven, whereby an image may be formed at an uneven density.